The present invention relates to semiconductor process endpoint systems. Endpoint systems determine the completion of a semiconductor process so that the semiconductor process can be halted.
Plasma etching endpoint systems detect and analyze light emitted from the formed plasma. As the etching step is completed, the composition of the emitted light from the plasma changes. These changes can be used to detect the endpoint of the plasma etch. Examples of such plasma etch endpoint systems include Koshimizu U.S. Pat. No. 5,322,590; O""Neill U.S. Pat. No. 5,308,414; and Dimitrelis U.S. Pat. No. 5,405,488.
Another type of etch is a xe2x80x9cliquidxe2x80x9d or xe2x80x9cwetxe2x80x9d etch. In a wet etch, a liquid etchant is placed on a wafer to etch away unwanted material, such as in a patterning step. Because wet etching systems do not use a plasma, no emitted light is produced that could be used to determine an endpoint.
One type of endpoint detection for an etch of a thin layer is an interferometric technique. When an etched layer has a index of refraction that is significantly different from an underlayer, the total reflectance is highly dependent on the thickness of the top layer as a function of wavelength.
For a given wavelength, some top layer thicknesses produce constructive interference of reflected light and some top layer thicknesses cause destructive interference of reflected light. A rainbow-colored oil slick on a puddle is a everyday illustration of a similar effect. Areas of oil with different thicknesses preferably reflect different wavelengths.
For single wavelength interferometry, typically, only a narrow wavelength range of light is reflected off of the substrate. As the top layer is etched away, the change in the top layer thickness causes oscillations in the graph of detected intensity versus time. When the top layer is removed, the detected oscillations end.
A disadvantage of the interferometric method is that a significant difference between the index of refraction of the etched layer and the index of refraction of the underlayer is required to obtain a good signal-to-noise ratio. For this reason, this method is inappropriate for use with many wet etch processes. For example, a deposited silicon dioxide layer has an index of refraction that is within one percent of the index of refraction of a thermally-grown silicon dioxide layer. The oscillations in the reflected light intensity for an etch of deposited silicon dioxide over grown silicon dioxide would be undetectable due to noise caused by environmental and process fluctuations. Intense background lighting is an example of an environmental fluctuation. Process fluctuations can include wafer chuck rotation, oscillations of the chemical dispenser, and non-uniform etching rates.
For the above reasons, it is desired to have an improved system for monitoring the endpoint of semiconductor process.
In one embodiment of the present invention, the endpoint of a semiconductor process is detected by identifying a sudden slope change in an intensity indication produced using light reflected off of a semiconductor wafer. The detection of a slope change does not require a top layer to have a different index of refraction from the underlayer. For example, for an etch of deposited silicon dioxide over thermally grown silicon dioxide, it has been found that the intensity of detected light reflected off of a wafer has a slope change near the endpoint.
In another embodiment of the present invention, light over a relatively wide range of wavelengths is reflected off of a wafer surface during a semiconductor process. Reflecting a relatively wide range of wavelengths off of a wafer to determine the endpoint has a number of advantages. First, this allows for two or more detectors to monitor two or more different wavelength ranges of reflected light. Monitoring light in two or more different ranges allows for an indication to be produced which is a function of multiple reflected light intensities. For example, the detected light intensity for a first wavelength range may have a different rate of change than a detected intensity for a second wavelength range as the top layer is etched away. An indication which is a ratio of the second intensity value over the first intensity value can remove some of the process noise and variability and produce a steeper slope function to give a better indication of the endpoint than either of the two intensities alone.
Since the light source produces a wide range of wavelengths, the system can provide the user with flexibility in selecting desired wavelength ranges to monitor during the process. The system of the present invention allows the selection of wavelength ranges that are particularly material dependent and thus repeatable for a given process.
The detected intensity of reflected light of a wafer material forms a repeatable xe2x80x9cfingerprintxe2x80x9d of the material. Further, the material""s xe2x80x9cfingerprintsxe2x80x9d can be used as a baseline to determine information concerning the process.
The prior art use of a narrow bandpass filter to filter the light from the light source restricts the wavelengths of light reflected off the wafer to a narrow range and thus prevents some advantages of the present invention. The light source that produces a wide range of wavelengths need not be a broadband light source. In fact, a preferred embodiment of the present invention uses a light source, such as a tungsten/mercury light bulb, that produces multiple emission peaks over the relatively wide wavelength range. The reflections off of the wafer at these emission peaks produces a more pronounced change in signal.
In one embodiment of the present invention, the change in the detected intensity of the reflected light versus time is the result of a change in the composition of the liquid etchant. While a top layer is being etched, substances such as products and by-products of the etch are added to the liquid etchant. These substances can affect the detected intensity of reflected light. Once the endpoint of the process occurs, these substances are not introduced into the liquid etchant at such a high rate and the composition of the etchant changes. Thus, the endpoint of the etch can be determined from a change in the detected intensity of reflected light. This effect can be used for etches in which the top layer has a similar index of refraction as the underlayer.
In another embodiment of the present invention, the change in the detected intensity of the reflected light versus time is the result of a different surface roughness or porosity of the etched layer compared to the underlayer. A rough surface will reflect light differently than a smooth surface. For this reason, the endpoint of the etch can be determined from a change in the detected intensity of reflected light even when the top layer has a similar index of refraction as the underlayer.
Another embodiment of the present invention involves the arrangement of the optical fibers. In a preferred embodiment, optical fiber(s) operably connected to the light source are surrounded by optical fibers operatively connected to the detectors. This allows for an efficient detection of light. For light incident straight down on a flat surface, the highest level of reflections tend to be near the source optical fibers. Arranging the detector optical fibers about the source optical fiber(s) allows for an efficiently detected signal. A preferred embodiment of the present invention arranges the detector optical fibers in a ring around the source optical fibers. If more than one detector is used, the detector optical fibers for each detector are preferably arranged relatively evenly about the ring for an even pickup of light from the same wafer surface area.
An additional embodiment of the present invention involves the use of multiple different types of detectors. Each type of detector can have a different operating wavelength range. A display of detected light intensity over the combined wavelength range can then be produced. The user can select wavelengths within the combined wavelength range for monitoring during the semiconductor process.
Another embodiment of the present invention concerns the detection of an endpoint of an etch of a deposited dielectric, such as a silicon dioxide layer formed in a chemical vapor deposition using tetra-ethyl-ortho-silicate (TEOS) over a grown dielectric such as a thermally-grown silicon dioxide. The endpoint of this etch cannot be accurately determined by an interferometric system. As discussed above, the index of refraction of a deposited silicon oxide is within one percent of the index of refraction of the grown silicon dioxide. By reflecting a wide range of wavelengths off of the wafer, it was found that the reflections at certain wavelengths are highly dependent on the process completion. For example, there are significant reflected light intensity changes at narrow wavelength range about 586 nm. A display of wavelength versus detected intensity before and after an etch allows for useful wavelength ranges to be found and selected by the user. These wavelength ranges can be monitored during the semiconductor process to determine an endpoint.
An etch of a silicon nitride layer over a deposited or thermally-grown oxide is another etch where the top layer has an index of refraction similar to the underlayer. The system of the present invention is particularly valuable to detect an endpoint of that etch as well.
Still another embodiment of the present invention concerns the detection of an endpoint of an etch of anti-reflective aluminum. Anti-reflective aluminum is typically positioned over polysilicon, or dielectric layer. Because of the lack of reflections from the anti-reflective aluminum, it is hard to detect an endpoint for such an etch. In the present invention, since a wide range of wavelengths are reflected off of the wafer, the user can select those wavelength ranges that are affected by the removal of such a layer.